The present invention relates to the treatment of spent hydrofluoric acid etching solutions used for etching silicate glasses. Such etching solutions are utilized in a number of glass finishing operations, including incandescent lamp bulb frosting, television faceplate polishing, and the chemical machining of glass.
Spent hydrofluoric acid solutions contain, in addition to substantial quantities of hydrofluoric acid, various by-products of glass dissolution. These include the dissolved metallic cations of lithium, aluminum, zinc, calcium and the like as well as complex anions containing silicon and aluminum. Among the most troublesome solution constituents from the standpoint of etching efficiency are siliceous compounds such as fluosilicic acid (H.sub.2 SiF.sub.6) which change the etching rate of the solution depending upon the composition of the glass. Fluosilicic acid also appears to participate in the formation of fluoride sludges and other deposits which clog the plumbing used to circulate the solution in the etching process.
The depletion of hydrofluoric acid from the etching solution in use can be counteracted by periodic replacement. However, the useful life of the solution is still limited by the buildup of fluosilicic acid and other glass dissolution by-products therein. Solution life could be extended significantly if an economic means for removing fluosilicic acid from the solution could be developed.
Reference to the use of ion-exchange resins to perform analytical and other separations of metals contained in hydrofluoric acid solutions is found in the literature. For example, separation of the dissolved metallic elements vanadium, scandium and arsenic from titanium, germanium and gallium through the ion-exchange treatment of hydrofluoric acid solutions of these metals is reported by U. Schindewolf and J. W. Irvine, Jr. in Anal. Chem. 30, 906 (1958). In carrying out these separations, a strong anion exchange resin is used, and the elution of anionic fluoride complexes of certain of these metals is controlled by varying the hydrofluoric acid concentrations of column influents.
A survey of the adsorption characteristics of a large number of different anionic metal fluoride complexes on a strong anion exchange resin was conducted by J. P. Faris, as reported in Anal. Chem. 32, 520 (1960). A considerable range of hydrofluoric acid concentrations was utilized, but not all elements were included in the survey.
The use of a strong cation exchange resin to separate metal cations through a hydrofluoric acid elution process is described by J. S. Fritz, B. B. Garralda and S. K. Karraker in Anal. Chem. 33, 882 (1961). These authors attribute the rapid preferential elution of certain metals by weak hydrofluoric acid solutions to the formation of anionic fluoride complexes thereof in the elution process.
None of the foregoing authors actually dealt with the problem of removing complex silicofluoride anions from strong hydrofluoric acid solutions, or the suitability of ion exchange resins for achieving this result. It is a principal object of the present invention to provide an ion exchange process which can economically be used to purify spent hydrofluoric acid solutions by removing these ions therefrom.